The work extended our ongoing study of interactions between the retinal glial Muller cell and T-lymphocytes. In an in vitro co- culture system, Muller cells had been shown to exert a profound inhibitory influence on the proliferation of T-helper cell lines through a membrane bound factor. Investigations of the nature of the inhibitory moiety revealed that it was sensitive to proteinase. Further studies showed that the expression of the factor on the surface of Muller cells could be suppressed by glucocorticoids.